Efforts to reduce the environmental impact resulting from combustion of hydrocarbon fuels continues to place increasingly stringent requirements on acceptable levels of total bound sulfur content in motor fuels. Recently issued federally mandated guidelines will eventually pose a significant challenge to present ASTM methods for laboratory and on-line analysis of these products with desired concentrations even lower than presently imposed. As a result, improved or alternative methods need to be developed in order to keep pace with strict requirements being placed on these fuels. The following describes a proposed improved method and the associated predictive results.
At present, the X-Ray Fluorescence (XRF) method has limited ability to provide repeatable analysis of sulfur content below present guidelines. Flame Photometric Detection (FPD) and Sulfur Chemiluminescence (SCD) methods possess much greater sensitivity, but suffer from a variety of problems. Both methods require the use of hydrogen, a hazardous gas, which increases the cost and complexity of these systems in order to meet safety requirements. The FPD method also has a characteristic non-linear response to sulfur requiring multi-point calibration and/or sulfur addition to approximate linear results. Chemiluminescence analysis additionally requires a stable vacuum source and ozone generator, increasing system complexity and making it notorious for high maintenance and long-term stability issues.
The simplest, most practical low-level sulfur analysis technique that has been widely used and proven over many years of utilization is the UV Fluorescence method. This method involves the optical excitation of sulfur dioxide and detecting a longer wavelength, secondary emission continuum. Total sulfur content in fuels is determined by complete combustion of all hydrocarbon and sulfur species, which are oxidized to CO2, H2O and SO2.
Originally developed UV fluorescence detectors were continuous types, but sensitivity of these detectors is somewhat limited due to inherent background noise or baseline instability, which is partially due to lamp intensity variations from plasma instabilities within the UV excitation lamp. Both cadmium and zinc excitation lamps have been successfully applied utilizing the 228 nm and 214 nm emission lines respectively, but the present accepted ASTM method favors the zinc lamp due to minimal fluorescence quenching influence from variations in the moisture content of the sample.
In an attempt to further decrease lower detectable limits, the Pulsed UV Fluorescence (PUVF) method was developed. The operating principle of PUVF is very similar to continuous type UV Fluorescence detectors, except that it uses a xenon flashlamp as the excitation source. Since the light is electronically “pulsed,” signal-to-noise ratios are improved and a lower level of detectability can be routinely achieved. However, since flashlamp intensity varies substantially from pulse-to-pulse, flash intensity is electronically normalized utilizing a separate light detector and induced fluorescence is averaged over a period of time. The PUVF detector is ideally suited for atmospheric monitoring and similar type applications where typically encountered SO2 concentrations generally change more slowly with time. Unfortunately, the pulsing excitation source also renders the PUVF a poor detector for higher speed detection requirements, such as total sulfur peak integration or sulfur speciation applications by chromatographic analysis.
In addition, the complexity of additional electronics required for flashlamp operation, signal synchronization and subsequent data averaging yields a detector that is considerably more complex and costs more than the continuous type UV detectors.
Although some additional improvement to accepted UV fluorescence detection methods may be realized by optimizing detector design, inherent limitations prevent the present methods from any quantum increase in sensitivity. Most efforts to further enhance sensitivity are primarily applied to reduction of background noise through improved detector geometry and careful selection of optical components to maximize excitation and fluorescence signals. Since several manufacturers of SO2 fluorescence detectors have attempted to push the limits of detection for many years, it is this author's opinion that significant improvement to present detection levels is unlikely utilizing present methods.
There exists, therefore, a need for improved UV fluorescence detection of sulfur dioxide having enhanced sensitivity.
A new excitation source can yield a significant improvement to present SO2 sensitivities achieved with ASTM accepted standards utilizing UV Fluorescence methods. This new excitation source, or Excimer lamp, creates a high intensity UV emission that possesses higher spectral purity than traditional zinc or cadmium lamps.
Excimer lamps are barrier-discharge devices based on the “excited dimer” principle and contain either an inert fill gas or a specific halogen and inert fill gas mixture. An alternating high voltage field is applied to electrodes separated by an insulating medium or dielectric and induces the generation of microscopic internal filament discharges, which excite fill gas atoms into briefly forming an excited molecular species. These short-lived transient molecules rapidly dissociate and release photons of specific wavelengths directly related to the quantum energy lost in the transition to ground state.
A feature of the present invention is to provide a UV fluorescence detection system and method having emission that is caused by excitation at a wavelength that correlates with absorption bands of the species under investigation.
A feature of the present invention is to provide a UV fluorescence detection system and method having an excitation wavelength that correlates with peak absorption bands of SO2 which create maximum fluorescence intensity.
Another feature of the present invention is to provide a UV fluorescence detection system and method possessing maximum fluorescence intensity at or near an excitation wavelength of 222 nm so as to correlate with absorption bands that induce the greatest fluorescence emission.
Another feature of the present invention is to provide a UV fluorescence detection system and method utilizing a dielectric barrier-discharge excimer lamp.
Another feature of the present invention is to provide a UV fluorescence detection system and method utilizing a krypton-chloride (KrCl*) excimer mixture.
Another feature of the present invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral emission purity to decrease the effects of background measurement limitations.
Another feature of the present invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral emission purity so as to increase signal to noise ratio.
Yet another feature of the invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral purity so as to possibly eliminate the need for an excitation source filter for some applications.
Another feature of the present invention is to provide a UV fluorescence detection system and method that can be operated in either continuous or pulsed modes of operation.
Yet another feature of the present invention is to provide a pulsed UV fluorescence detection system and method for further lowering minimum detectable levels or concentrations.
Still another feature of the present invention is to produce SO2 fluorescence with higher rejection ratios to interfering nitric oxide (NO) than obtained with a zinc lamp.
Yet another feature of the present invention is to provide a pulsed UV fluorescence detection system and method that can be closed-loop-feedback controlled for more stable operation.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.